The battery of an implanted medical device should have a reliable, long life. Engineering progress in battery design has lead to substantial reductions in battery volume while maintaining, or improving upon, the device longevity. This reduction in battery volume has resulted in smaller and more comfortable devices for the patient.
Implantable cardiac defibrillators (ICDs) generally have monitoring and control circuitry to sense and analyze cardiac events, and electrical output circuitry to provide both pacing level and defibrillation level electrical energy to the cardiac tissue in response to the control circuitry. These ICD circuitry components, however, have different power requirements. While the monitoring and the pacing control and output circuitry require a continuous low-current supply over the life of the ICD, the defibrillation output circuitry require a high-current supply for very short time durations to enable the defibrillation electrical output circuitry to deliver a defibrillation level shock to the patient in a timely manner. Therefore, one of the challenges in designing ICDs is the integration of these circuitry components with vastly different power requirements into a ICD using a single current battery.
Typically, two lithium silver vanadium pentoxide batteries coupled in series have been used to achieve the high-current, high power density, requirements for the ICDs defibrillation output circuity. This high power capability battery design is suited for the high-voltage defibrillation output circuity which draws peak current drains on the order of amperes from the battery. By contrast, the low-current monitoring circuit, the logic control circuit, and any accompanying pacing output circuit of the ICD have peak battery demands in the microampere to milliampere range. The high power density batteries are typically regulated down to provide a stable low-current power supply to the monitoring and pacing control circuits over a wide range of potential battery loads. The down regulation of the power dense batteries is, however, an inefficient use of the depletable power source and thus a larger battery must be used to provide for the reliable operation of the ICD.
To resolve the dichotomy of combining high- and low-power circuits, the use of two separate batteries has been attempted. These "dual battery" systems provide two physically independent batteries matched to the separate power requirements of the ICD. U.S. Pat. Nos. 5,405,363 (Kroll et al.) and 5,439,482 (Adams et al.) describe the use of "dual battery" ICD systems. Each patent suggests that utilizing two batteries can result in a reduction of overall volume of the battery system as compared to a single battery system. While these patents describe reductions in ICD battery volumes, further reductions in battery volume and weight continue to be a goal of ICD designers.